Positron emission tomography (PET) is a technology of functional medical image detection. PET is based on the theory (Annihilation Coincident Detection, ACD) that a positron and an electron annihilate with each other to generate a pair of annihilation photons.
FIG. 1 is a diagram showing the pair of annihilation photons. The pair of annihilation photons with 511 keV energy have high probability to pass through an object. When the pair of photons are detected by detector pair of a detector around the object in a short time interval, it means a coincident event.
The short time interval is also called coincidence timing window or time window. The route of two photons being detected is called line of response (LOR). All coincident events occurred on LOR are listed in the counts of the detector. Finally, stacking all the LORs with different angles and different positions can get a Sinogram, and further to calculate and reconstruct all of the data to form a PET image.
During PET scanning, if more than three photons are detected in the same time window defined by a system, it's called multiple event which also comprises triple event. The information of multiple event doesn't be taken record because it is can not be determined that which position detected are the pair of annihilation photons occurring at, even can not be determined which photons are annihilation photons.
Therefore, in convention, almost data of triple event are abandoned to avoid erroneous determination. Besides, when the object includes higher activity, the probability of triple event becomes higher, the more data are abandoned. These data can not provide any useful help in the technology after being abandoned and it wastes much useful information.